Conventionally, two types of batteries are known: a capacity type battery and an output type battery. The capacity type battery is a low output large capacity battery suitable for a small electric current discharge over a long period of time. The output type battery is a high output small capacity battery suitable for a large electric current discharge over a short period of time. Japanese Patent Laid-Open No. 2004-364350 (JP '350) discloses a technique for realizing a downsized battery volume by combining the capacity and output type batteries in parallel. Further, Japanese Patent Laid-Open No. 2006-79987 (JP '987) discloses a technique for combining the capacity and output type batteries in an electric vehicle and for controlling the discharge of an electric current from one of the batteries according to a travel condition of the vehicle.
JP '350 and JP '987 respectively disclose a technique of discharging the electric current from a combination of two different types of batteries, but both are silent about the charging (i.e., storage) of electric energy. The two types of electric storage units (i.e., the capacity and output type batteries) may sometimes be installed in a building with a photovoltaic power generation device. In such case, depending on the amount of electricity generated by the photovoltaic power generation device and stored in the electric storage units, electric power not consumable by the building may be stored in the electric storage units or may be returned to a grid-connected power system by reverse flowing electric current. That is, when there are two types of electric storage units installed in the building, controlling the discharge of electric current from one of the electric storage units needs to be considered or scheduled.
Further, the price of a two-in-one type battery that has a capacity of both the capacity type battery and the output type battery may still be high, thereby rendering the use of such battery not feasible in terms of cost and forcing a user to use two types of batteries in parallel.